I. Field of the Invention
The present invention pertains generally to the field of communications, and more particularly to generating pilot strength measurement messages in a wireless communication system.
II. Background
In the field of wireless communications, several technology-based standards exist for controlling communications between a mobile station, such as a cellular telephone, Personal Communication System (PCS) handset, or other remote subscriber communication device, and a wireless base station. These include both digital-based and analog-based standards. For example, among the digital-based cellular standards are the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) Interim Standard IS-95 series including IS-95A and IS-95B, entitled xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System.xe2x80x9d Similarly, among the digital-based PCS standards are the American National Standards Institute (ANSI) J-STD-008 series, entitled xe2x80x9cPersonal Station-Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems.xe2x80x9d Other non-CDMA based digital standards include the time-division multiple access (TDMA) based Global System for Mobile Communications (GSM), and the U.S. TDMA standard TIA/EIA IS-54 series.
The spread spectrum modulation technique of CDMA has significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, issued Mar. 31, 1992, entitled xe2x80x9cSOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d and U.S. Pat. No. 5,109,390, issued Apr. 28, 1992, entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM, xe2x80x9d both assigned to the assignee of the present invention and incorporated by reference herein.
The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for cell-site and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM,xe2x80x9d Ser. No. 07/433,031, filed Nov. 7, 1989, also assigned to the assignee of the present invention. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
The aforementioned patents all describe the use of a pilot signal used for acquisition in a CDMA wireless communication system. At various times when a wireless communication device such as a cellular or PCS telephone is energized, it undertakes an acquisition procedure which includes, among other things, searching for and acquiring the pilot channel signal from a base station in the wireless communication system. For example, demodulation and acquisition of a pilot channel in a CDMA system is described in more detail in copending U.S. patent application Ser. No. 08/509,721, filed Jul. 31, 1995, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM,xe2x80x9d now U.S. Pat. No. 5,805,648, issued on Sep. 8, 1998, assigned to the assignee of the present invention and incorporated herein by reference. When more than one pilot channel can be acquired by the wireless communication device, it selects the pilot channel with the strongest signal. Upon acquisition of the pilot channel, the wireless communication device is rendered capable of acquiring additional channels from the base station that are required for communication. The structure and function of these other channels are described in more detail in the above referenced U.S. Pat. No. 5,103,459 and will not be discussed in detail herein.
The acquisition procedure for searching and acquiring the pilot channel signals from base stations has the purpose of detecting potential candidate base stations for handoff. The viable base station candidates can be divided into four sets. These sets are used to prioritize the pilots and increase the efficiency of searching. The first set, referred to as the Active Set, comprises base stations that are currently in communication with the mobile station. The second set, referred to as the Candidate Set, comprises base stations that have been determined to be of sufficient strength to be of use to the mobile station. Base stations are added to the candidate set when their measured pilot energy exceeds a predetermined threshold TADD. The third set is the Neighbor Set which is the set of base stations which are in the vicinity of the mobile station (and which are not included in the Active Set or the Candidate Set). And the fourth set is the Remaining Set that consists of all other base stations.
In an IS-95A communication system, the mobile station sends an autonomous Pilot Strength Measurement Message (PSMM) when the mobile station finds a pilot of sufficient strength that is not associated with any of the Forward Traffic Channels currently being demodulated or when the strength of a pilot that is associated with one of the Forward Traffic Channels being demodulated drops below a threshold for a predetermined period of time. The term pilot refers to a pilot channel identified by a pilot sequence offset and a frequency assignment. The mobile station sends an autonomous PSMM following the detection of a change in the pilot strength when one of the following conditions are met:
1. The strength of a Neighbor Set or Remaining Set pilot is found above the threshold (TADD).
2. The strength of a Candidate Set pilot exceeds the strength of an Active Set pilot by more than a threshold (TCOMP)xc3x970.5 dB, and a PSMM carrying this information has not been sent since the last Handoff Direction Message (HDM) or Extended Handoff Direction Message (EHDM) was received.
3. The strength of a pilot in the Active Set of Candidate Set has fallen below a threshold (TDROP) for greater than a predetermined time period (TTDROP), and a PSMM carrying this information has not been sent since the last HDM or EHDM was received.
TADD is threshold above which the received signal is of sufficient strength to effectively provide communications with the mobile station. TDROP is a threshold value below which the received signal energy is insufficient to effectively provide communications with the mobile station.
In an IS-95B communication system, the mobile station sends an autonomous PSMM according to one of two sets of rules as chosen by the base station. The first set of rules is the same as the rules specified in IS-95A. The second set of rules uses a dynamic threshold defined as:             T      DYN        =                            SOFT_SLOPE          8                xc3x97        10        xc3x97        log        ⁢                              ∑                          i              ∈              A                                ⁢                                    (                              Pilot                ⁢                                  xe2x80x83                                ⁢                                  Ec                  /                  Io                                            )                        i                              +              ADD_INTERCEPT        2              ,
where the parameters SOFT_SLOPE and ADD_INTERCEPT are specified by the base station and the summation is performed over all pilots in the Active Set. Ec/Io is the ratio of pilot energy per chip to the total received spectral density of noise and signals.
According to the second set of rules of IS-95B, the mobile station sends an autonomous PSMM if any of the following conditions occur:
1. The pilot strength of a Candidate Set pilot is found to be above TDYN, and a PSMM carrying this information has not been sent since the last EHDM or General Handoff Direction Message (GHDM) was received;
2. The pilot strength of a Neighbor Set pilot or Remaining Set pilot is found to be above max(TDYN, TADD/2);
3. The pilot strength of a Candidate Set pilot exceeds the strength of any Active Set pilot by TCOMPxc3x970.5 dB and is above TDYN, and a PSMM carrying this information has not been sent since the last EHDM or GHDM was received;
4. The handoff drop timer of an Active Set pilot has expired and a PSMM carrying this information has not been sent since the last EHDM or GHDM was received.
The rules according to IS-95A and IS-95B are designed for single carrier systems that use a 1.25 MHz channel on both the forward link and the reverse link. However, in a multi-carrier system, the mobile station receives the pilot channel of a base station on multiple carrier frequencies simultaneously. For example, a 3xc3x97/1xc3x97 multi-carrier system may use three 1.25 MHz channels on the forward link and one 1.25 MHz channel on the reverse link. Another example is a 3xc3x97/3xc3x97 multi-carrier system using three 1.25 MHz channels on the forward link and a 3.75 MHz channel on the reverse link. In either example, one can see short term fading that varies from one carrier frequency to another. In such a situation, the IS-95 rules that govern the autonomous transmission of a PSMM are inadequate in the presence of pilots on multiple pilot channels. Hence, there is a present need to determine when a mobile station transmits an autonomous PSMM according to the receipt of multiple pilot signals from base stations in multi-carrier systems.
The present invention is directed to a method and apparatus for generating an autonomous Pilot Strength Measurement Message (PSMM) by a mobile station in a multi-carrier wireless communication system, comprising the steps of receiving a plurality of pilots at a mobile station, wherein the plurality of pilots are transmitted from at least one base station; using a first pilot strength definition from a set of pilot strength definitions to determine a pilot strength associated with at least one of the plurality of pilots; checking a set of rules, wherein the set of rules is for generating the PSMM by manipulating the first pilot strength definition; and generating the PSMM for transmission from the mobile station.
In one aspect of the invention, differing pilot strength definitions are used in the set of rules by applying one pilot strength definition in one rule while applying a different pilot strength definition in another rule. In another aspect of the invention, the PSMM generated by the mobile station carries pilot strength information that was not used to generate the PSMM.